1. Field of the Invention
The present invention relates to a valve timing control device that controls the valve timing of an engine in a variable manner, and an engine device and a vehicle including such a control device.
2. Description of the Related Art
Many kinds of variable valve timing (VVT) mechanisms that control the opening/closing timings of an intake valve or an exhaust valve have been developed in order to improve fuel consumption, reduce toxic substances in exhaust gas, and achieve high power output in a target engine speed range.
Some of the variable valve timing mechanisms use an actuator such as a hydraulic cylinder and an electric motor. However, these actuators are expensive and the use of such an actuator increases the size of the variable valve timing mechanism.
In general, the space occupied by an engine in a motorcycle is smaller than that of a four-wheeled automobile and the like. There has been a demand for motorcycles that can be manufactured less costly. Therefore, there is a demand for more inexpensive and compact variable valve timing mechanisms for use in motorcycles. This is why the variable valve timing mechanisms including actuators as described above cannot be used in motorcycles.
A rotation phase generator has been suggested as a variable valve timing mechanism that can be made compact (see, for example, Japanese Laid Open Patent No. 09-324614 (Patent Document 1)).
In the rotation phase generator, an input member including two intermediate members is rotated with the rotation of the engine. When centrifugal forces acting on a weight portion of the two intermediate members is greater than the biasing force of a coil spring connecting these intermediate members, the rotation phases of the input member and an output member connected to a camshaft change, so that the valve timing changes.
With the rotation phase generator disclosed in Patent Document 1, the valve timing is controlled depending on the mechanical arrangement, and therefore the cost and size can be reduced.
However, the following problems associated with the above-described variable valve timing mechanism have been pointed out.
In the rotation phase generator disclosed in Patent Document 1, when the valve timing changes, centrifugal forces acting on the weight portion balances with the biasing force of the coil spring in a certain engine speed range of the engine. At that time, while the cam makes one rotation, a force (resistance force) in the forward or backward rotation direction constantly acts on the cam. The resistance force results from the elastic force of the valve spring and the inertial force generated by other valve system components.
If the engine speed of the engine is maintained within an engine speed range, the resistance force acts as a load in the positive direction relative to the centrifugal force of the weight during a prescribed period as the cam makes one rotation. During the other period, the resistance force described above acts as a load in the negative direction relative to the centrifugal force of the weight. Therefore, the state in which the centrifugal force acting on the weight and the biasing force of the coil spring are balanced cannot be maintained while the cam makes one rotation. In this case, the behavior of the weight becomes unstable.
As a result, the valve timing changes unstably, and the behavior of the valve becomes unstable, in other words, a phenomenon called hunting is caused.
Hunting gives rise to noises and degradation in the durability of the components. When the cam profile is changed by hunting, the performance and durability of the engine can be lowered in some cases.